Electrical static discharge is electrical discharge of static electricity that builds up on personnel or equipment, generated by interaction of dissimilar materials. Static sensitive devices, especially those of a printed circuit assembly (PCA), may be damaged or disturbed when they come into close proximity with electrostatic charged objects. This is also true when the static sensitive devices are packaged in a container (e.g., a plastic case) for shipping purposes or other purposes and the outside of the container comes in contact with electrostatic charged objects. Cases or containers having a seam or path, such as between a light pipe, or a switch or another actuator, and the case or container, are especially troublesome for static sensitive devices since the seam or path provides a conduit for electrostatic electricity to the static sensitive devices.
There are a number of conventional solutions to prevent damage to static sensitive devices that are proximately disposed to a light pipe. One method for increasing electrostatic discharge immunity involves increasing the linear distance between the electrostatic discharge “charged” object and the static sensitive device on a printed circuit assembly. The linear distance is increased by adding an additional flat surface of plastic to the light pipe such that the new surface is oriented perpendicular to the axis of the light pipe. As the size/diameter of the flat surface is increased and the corresponding linear distance for a discharge from an electrostatic discharge device to travel is increased, the voltage needed to breakdown between the charged object and the printed circuit assembly is increased. The primary disadvantage in the use of a wide flat surface around a light pipe to increase the electrostatic discharge immunity is the requirement for a large space around the light pipe or actuator. For small products and products with a light pipe located in a corner or adjacent to another mechanical or electrical component, there may not be adequate clearance around the light pipe to add a large enough flat surface to provide a meaningful increase in electrostatic discharge immunity protection.
Another method of increasing electrostatic discharge immunity involves removing or reducing the “air-gap” between the light pipe and the product case. The space between the light pipe and the product case may be removed/reduced by any one of the following three options: (i) glue may be applied to the seam between the product case and the light pipe at the time of assembly; (ii)the plastic of the light pipe maybe attached via ultra-sonic welding causing the plastic of the product's case to melt together with the plastic of the light pipe; (iii) the light pipe and the plastic product case may be manufactured to precision tolerances and assembled to obtain what is regarded as a non-ionizable air gap of less than about 10 um. The primary disadvantage of these air-gap reduction alternatives is centered on increased material and labor costs. More specifically, the glue alternative approach increases the manufacturing cost for both labor and material in addition to loosing the flexibility of rework and decreased cosmetic yields. The ultra-sonic welding approach increases labor cost by requiring complex machines and training and additionally reduces flexibility of reworking products if required. The precision-tolerance approach which reduces the air-gap to less than 10 um results in substantial increased cost in manufacturing by injection molding, as well as the labor to manufacture the parts.
The options mentioned above fail for parts that need to be moving. An example of a situation in which a moving member passes through the structural member is a PCB-mounted switch having a plastic rod, as actuator element, that extends through the plastic enclosure.
In this case, neither glue nor welding can be applied. Creating and maintaining a very narrow gap under use a gap between the moving actuator and the plastic enclosure is very difficult, such that other solutions must be sought.